This invention relates to transducers and more particularly to phased array transducers for use particularly in the medical diagnostic field.
Ultrasound machines are often used for observing organs in the human body. Typically, these machines contain transducer arrays for converting electrical signals into pressure waves and vice versa. Generally, the transducer array is in the form of a hand-held probe which may be adjusted in position to direct the ultrasound beam to the region of interest.
FIG. 1 illustrates a prior art transducer array 10 for generating an ultrasound beam. Typically, such an array may have 128 transducer elements 12 in the azimuthal direction. Adapted from radar terminology, the x, y, and z directions are referred to as the azimuthal, elevation, and range directions, respectively.
Each transducer element 12, typically rectangular in cross-section, includes a first electrode 14, a second electrode 16 and a piezoelectric layer 18. In addition, one or more acoustic matching layers 20 may be disposed over the piezoelectric layer 18 to increase the efficiency of the sound energy transfer to the external medium. The electrode 14 for a given transducer element 12 may be part of a flexible circuit 15 for providing the hot wire or excitation signal to the piezoelectric layer 18. Electrode 16 for a given transducer element may be connected to a ground shield return 17. The piezoelectric layer 18 is metalized on its top and bottom surfaces and the matching layer 20 is also metalized on all surfaces so that electrode 16 which is in physical contact with the matching layer 20 is electrically coupled to a surface of the piezoelectric layer 18 by the metallization.
The transducer elements 12 are disposed on a backing block 24. The backing block 24 may be highly attenuative such that ultrasound energy radiated in its direction (i.e., away from an object 32 of interest) is substantially absorbed. In addition, a mechanical lens 26 may be placed on the matching layer 20 to help confine the generated beam in the elevation-range plane and focus the ultrasound energy to a clinically useful depth in the body. The transducer array 10 may be placed in a nose piece 34 which houses the array. Examples of prior art transducer structures are disclosed in Charles S. DeSilets, Transducer Arrays Suitable for Acoustic Imaging, Ph.D. Thesis, Stanford University (1978) and Alan R. Selfridge, Design and Fabrication of Ultrasonic Transducers and Transducer Arrays, Ph.D. Thesis, Stanford University (1982).
Individual elements 12 are electrically excited by electrodes 14 and 16 with different amplitude and phase characteristics to steer and focus the ultrasound beam in the azimuthal-range plane. An example of a phased array acoustic imaging system is described in U.S. Pat. No. 4,550,607 issued Nov. 5, 1985 to Maslak et al. and is specifically incorporated herein by reference. U.S. Pat. No. 4,550,607 illustrates circuitry for combining the incoming signals received by the transducer array to produce a focused image on the display screen. When an electrical signal is imposed across the piezoelectric layer 18, the thickness of the layer changes slightly. This property is used to generate sound from electrical energy. Conversely, electrical signals are generated across the electrodes in contact with the piezoelectric layer 18 in response to thickness changes that have been imposed mechanically from sound waves reflected back to the piezoelectric layer 18.
The pressure waves generated by the transducer elements 12 are directed toward an object 32 to be observed, such as the heart of a patient being examined. Each time the pressure wave confronts tissue having different acoustic characteristics, a wave is reflected backward. The array of transducers may then convert the reflected pressure waves into corresponding electrical signals.
For the transducer shown in FIG. 1 the beam is said to be mechanically focused in the elevation direction. The focusing of the beam in the azimuthal direction is done electronically by controlling the timing of the transmissions of each transducer element. This may be accomplished by introducing appropriate phase delays in the firing signals.
Reflected energy from a particular location in the imaging plane is collected by the transducer elements. The resultant electronic signals from individual transducer elements are individually detected and reinforced by introducing appropriate delays. Extensive processing of such data from the entire imaging phase is done to generate an image of the object. Such an image is typically displayed on a CRT monitor.
Sometimes it is desirable to image particular features to the exclusion of others. For example, it may be desirable to image the flow of blood in a patient to the exclusion of the surrounding organs and muscles. Introducing contrast agents into the patient""s bloodstream allows the imaging of the blood stream. Contrast agents may be in the form of a solution or suspension of microbubbles or agents that produce microbubbles. The use of contrast agents provides selective evaluation of the signal components affected by the materials or media which have been introduced. This has the advantage that selective representation of the region filled with those agents is possible without finding the difference between two or more conditions recorded before and after application of the materials or media.
Nonlinear contrast agents are described for example by V. Uhlendorf, et al., in xe2x80x9cNonlinear Acoustical Response of Coated Microbubbles in Diagnostic Ultrasoundxe2x80x9d (1995) Ultrasonic Symposium, pp. 1559-1562). Such agents possess a fundamental resonant frequency. When they are insonified with high intensity ultrasonic energy at this fundamental frequency, they radiate ultrasonic frequency at a harmonic of the fundamental frequency. Such contrast agents are often used to highlight regions containing blood loaded with the contrast agent. For example, in the case of a blood-filled chamber of the heart, the borders of the chamber can be distinguished more easily when contrast agent is used. Since the contrast agent generates harmonic ultrasound energy, echoes from tissue (containing no contrast agent) at the fundamental frequency may be eliminated or reduced by filtering at the receive beamformer. Because most transducers operate in the half wavelength resonance mode they are not able to effectively receive energy at a second harmonic frequency since at the second harmonic frequency, the transducer elements are approximately one wavelength thick. This causes the charge generated on the two halves of the transducer element to be out of phase with each other which results in a cancellation or a null.
A wideband transducer can be operated to transmit pressure waves at one frequency and receive second harmonic frequency signals reflected back. FIG. 2 is a graph illustrating the transmit response from a transducer having a wide bandwidth, for example 70%. A bandwidth of 70% means that the bandwidth measured between the lower frequency at which the sensitivity is xe2x88x926dB with respect to the maximum sensitivity attained over the useful frequency range of the transducer and the upper frequency at which the sensitivity is xe2x88x926dB with respect to the maximum sensitivity is 70% of the center frequency where the center frequency is defined as the average of the lower and upper xe2x88x926dB frequencies. Using a transducer with a center frequency fc of 4.5 MHz, an ultrasound wave can be transmitted at 2/3 fc or 3 MHz and received at 4/3 fc or 6 MHz.
While energy may be transmitted and received within the transducer""s available bandwidth, there are several disadvantages associated with using a wideband transducer in such a manner. Because transducer bandwidths are typically 75% and less, it is necessary to work near the edges of the transducer""s bandwidth in order to transmit at one frequency and receive at another. This results in lower sensitivity and undesirable filtering effects on the lower edge of the spectrum in transmit and on the upper end of the spectrum on receive as will be illustrated in the graphs of FIGS. 3 and 4. Ideally it is desirable to operate near the center of the available bandwidth for maximum sensitivity and spectral purity.
FIG. 3 is a graph illustrating the transducer transmit response (in dashed line) and the desired transmit response and receive response centered at 2/3 fc and 4/3 fc respectively (in solid line). FIG. 4 is a graph illustrating the filtering effect of operating a wide bandwidth transducer near the edges of its bandwidth. The desired transmit and receive response are shown in solid line and the distorted, filtered transmit and receive response are shown in dashed line. It can be seen that a portion of the desired spectra has been removed by the filtering effect of the transducer.
It is thus desirable to provide a transducer structure that can be optimized to transmit pressure waves at one frequency and receive energy at another frequency. More particularly, it is desirable to provide a transducer that can generate pressure waves at a first fundamental frequency and receive pressure waves at a second harmonic frequency.
According to a first aspect there is provided an ultrasound transducer probe for transmitting an ultrasound beam into an area of examination and receiving signals reflected from said area of examination. The ultrasound transducer probe includes a first layer having a first electrode on one side of the first layer and a second electrode on an opposite side of the first layer. The first layer emits an ultrasound beam when a signal is applied to the first and second electrodes and the first layer develops a signal across the first and second electrodes upon receipt of an ultrasound beam reflected back from the area of examination. A second layer is disposed on the first layer. The second layer has a third electrode on one side of the second layer and a fourth electrode on an opposite side of the second layer. The second layer emits an ultrasound beam when a signal is applied to the third and fourth electrodes and the second layer develops a signal across the third and fourth electrodes upon receipt of an ultrasound beam reflected back from the area of examination. Means for isolating the signal developed across the second layer are provided as well as at least one tuning element.
According to a second aspect, there is provided a method for imaging an area of examination by transmitting an ultrasound beam into the area of examination and receiving signals reflected back from the area of examination. The method includes the steps of providing an ultrasound transducer having at least a first layer and a second layer, disposed on the first layer, transmitting an ultrasound beam by applying a signal across both the first and second layers, receiving signals generated across the first layer while isolating signals generated across the second layer, and filtering one of the transmit and receive signals as a function of the isolation.
According to a third aspect, there is provided a method for imaging an area of examination by transmitting an ultrasound beam into the area of examination and receiving signals reflected back from the area of examination. The area of examination is substantially free of contrast agents during an entire imaging session. The method includes the steps of providing an ultrasound transducer having at least a first layer and a second layer, disposed on the first layer, transmitting an ultrasound beam by applying a signal across both the first and second layers, and receiving signals generated across the first layer while isolating signals generated across the second layer.
The invention itself, together with further objects and attendant advantages, is defined by the following claims and will best be understood by reference to the following detailed description, taken in conjunction with the accompanying drawings.